1. Field of the Invention
The present invention relates to an architecture for a bi-directional fiber optical link.
2. Discussion of the Known Art
The aerospace operating environment is hostile and it imposes many constraints on the engineering and design of avionics systems. Most critical is the stringent operational temperature range of from −55° C. to +125° C. Optical devices are especially susceptible to temperatures that might lead to a link failure or other catastrophic loss, and the use of cooling or heating units only increases system size, weight and power (SWAP), as well as cost. As a result, advanced jet fighters like the F35 still use IEEE 1394B protocols for electrical data distribution over copper (Cu) wire in their flight and mission control systems.
Optical networking technologies are expected to revolutionize next generation avionics and naval communication systems. Optical systems offer substantial improvements over Cu wire systems in reliability, size, weight, power efficiency, cost, security, immunity to electromagnetic interference, and networking capability. Skyrocketing fuel prices and an ever increasing demand for bandwidth make it necessary to build avionics platform networks with reduced SWAP but are nevertheless flexible, scalable and upgradeable with minimal installation and lifetime operation costs. With reduced SWAP, an air fighter can carry more fuel and ordinance over greater distances.
Optical networks are potentially capable of meeting all of the above requirements. A so-called Requirement of Optical Networks in Avionics (RONIA) program at the Defense Advanced Research Projects Agency (DARPA) estimates a total backbone capacity of about 1.4 Tb/s with about 400 nodes and over 500 links for an air fighter. The data rate per node is expected to be 1 Gb/s or greater. For example, a significant weight reduction may be achieved by eliminating heavy Cu cables. Optical data networks also offer a large suite of other benefits including resistance to electromagnetic interference (EMI) and unauthorized tapping, very large bandwidth, protocol transparency, low loss, low crosstalk with more than 40 dB isolation, corrosion resistance, and no radiation, fire ignition or electrocution hazards.
Despite their widespread use in commercial and residential deployments, optical networks currently deployed in avionics largely comprise point-to-point multimode fiber links operating at a 850 nm wavelength and use so-called Fiber Channel protocols for storage. As a result, electronic communications are still carried out mainly over Cu wiring employing ARINC 429, Avionics Full Duplex Switched Ethernet (AFDX), IEEE 1394 and US MIL-STD-1553B Standards. The IEEE 1394 protocol is used on the Joint Strike Fighter for vehicle management systems, and it supports a 400 Mb/s data transfer rate. AFDX, which is ARINC 664 (Part 7), presently supports a 100 Mb/s data rate.
Current aerospace optical fiber back plane networks can be complicated and costly to maintain and repair, and typically require built in tests (BIT) and fault diagnosis procedures in their construction. Ideally, there should be a minimal number of different spare parts needed to service the network, and replacements should preferably be carried out at a module level with ease of access.
System sensors, radar, RF antennas and cameras are the eyes and ears of an aircraft. They are sources of raw signals or data that need to be processed by an integrated core processor (ICP) located remotely from the various sources, and analyzed by the flight crew and/or other subsystems. FIG. 1 shows typical communication networks and systems within an airborne platform. The networks include, for example, networks for navigation, RF communications, tactical response, munitions control, identification and surveillance, electronic warfare, and storage. The platform also typically has a flight control and vehicle management system. Each network or system may use a different communication protocol and operate essentially independently. Communications to and from the platform are typically carried by RF waveforms, while flows of data within the platform between the various sources and the ICP may be either RF or digital baseband, with increasing movement toward the latter. The data may be continuous, random, or bursty.
In a data centric system, the core of an integration of the various platform systems should preferably be a unified network or data bus that will support transparent operation of a variety of otherwise incompatible protocols of the different systems. Optical networks can support the various protocols whether analog or digital. The ability of a core integrating network to support both analog and digital signals would provide a significant gain in SWAP by eliminating the present need for a separate RF cabling infrastructure.
U.S. Patent Application Publication No. 2004/0062553 (Apr. 1, 2004) describes a bidirectional optical link between first and second data units using a single optical source. In the disclosed embodiment, the first data unit is a transmit/receive unit associated with an aircraft. The second data unit is a ground terminal including a modulator/optical receiver system. A splitter element in the optical receiver system receives a modulated optical signal from an optical source in the first data unit. The incoming optical signal is split into a received portion and an outgoing portion. The received optical portion is detected and converted to an electrical signal. A return modulator element modulates the outgoing optical portion and transmits same to the first data unit. The modulation of the outgoing optical portion allows the link to use a single shared optical source, according to the '553 publication.
A full-duplex optical transmission link is disclosed in L. D. Westbrook et al., “Simultaneous bi-directional analogue fiber-optic transmission using an electro-absorption modulator”, Electronics Letters, vol. 32, no 19 (Sep. 12, 1996), at pages 1806-07. A downlink laser transmitter and a photodetector receiver are provided at the head end of an optical link consisting of a downlink fiber and an uplink fiber. A single electro-absorption (EA) device is used as a simultaneous photodetector receiver and modulator-transmitter at a remote end of the link. A duplexer is coupled to the EA device at the remote end, and full-duplex communication between the transmitters and the receivers at both ends of the link may be accomplished, as reported in the article.